Usually very hot, probably in the 30's. It does depend where in Greece/Greek Islands you are, some places may have a breeze which helps a bit. I've noticed that travel brochures no longer print a little chart of predicted temps in different months because with global warming it is no longer consistent.
You could google Temperatures in August in Greece and see what you get back. As long as you don't take it as definite, we have experienced a freak heatwave in Kefalonia two years ago, mid September. We thought it would be around the mid 20's, in fact we had two weeks of 38 degrees! On the plus side, the sea was lovely and warm.
girly - there was hardly a drop in temperature right through the night where we were!!! We attempted the beach but couldnt put our feet down even on the wooden paths to the sea LOL. The same year the buses and taxi's stopped for a while and nobody would take any tourists to Lindos because its always much hotter there!!! We were 'shade chasing' all day LOL. Last year, in August, we didn't see one cloud on any of the 14 days we were there!
Nothing would stop me from going back to Rhodes as often as possible though - we did a lot of 'I like it hot, but not this hot!!' (Peter Kays line) but it was glorious!!